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Spark plug question


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I am in process of engine tune and am new to BMWs  coming from air cooled world of VWs and Porsche. So I removed the plugs and they were all carbon fouled which I expected.They are the NGK plugs that are stock to the engine number 6 I believe.What kind of surprised was the the threads.They were also covered in carbon at least a good bit of them were.My question is how far into the combustion chamber go these plugs project. I always thought there really wasnt supposed to be exposed threads . It seems to me one of the potential issues would be removing a plug with carbon on the threads from being exposed in CC. Am I wrong here?

'67 Derby Grey VW Beetle

'76 Inka BMW 2002

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15 minutes ago, VWScott said:

how far into the combustion chamber go these plugs project. I always thought there really wasnt supposed to be exposed threads


They go in there quite a bit. Pretty typical for several threads exposed.

 

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10 minutes ago, visionaut said:


They go in there quite a bit. Pretty typical for several threads exposed.

 

be great to see a pic of how many threads are actually exposed.

'67 Derby Grey VW Beetle

'76 Inka BMW 2002

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2 minutes ago, e31fan said:

You could count the threads on that photo then on your plug to get a pretty good idea

yeah thats about my estimate as well.Doesnt that seem like a lot of the plug threads to be exposed.I've always seen plugs to just end in the CC with very little exposed.It seems like if you got crud on the threads it would make a mess while backing them out.

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'67 Derby Grey VW Beetle

'76 Inka BMW 2002

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well, a spark plug only has one thread, so...

 

And no, there shouldn't be much, if any, thread showing in the cylinder.

Ideally, just the unthreaded nose.

However, what you'll find is that the thread doesn't seal into the last few turns of its mating thread in the

head all that well, so the discoloration you're seeing isn't that unusual.

 

With a BP6ES, anyway.   

 

It's perfectly permissible to use shims under the base of the plug to get the protrusion exactly correct-

but most of us just shim to keep the ground strap from shrouding the spark from the center of the combustion chamber.

 

t

 

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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2 hours ago, VWScott said:

be great to see a pic of how many threads are actually exposed.

 

IMG_20240403_202200972.jpg

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4 hours ago, VWScott said:

What kind of surprised was the the threads.They were also covered in carbon at least a good bit of them were.

 

4 hours ago, VWScott said:

plugs.jpg

Colored threads are not from exposed in the combustion chamber. 

 

Below is taken from the plug reading guide I have.

 

"The threaded portion of the plug gives you the heat range, look at the threads you'll see that a few
toward the tip are a dull burnt looking color the rest are black and shiny. You want about 2 threads
showing the heat on the end of the plug and the rest of the threads to be shiny, this plug is impossible
to read because of the oil mess. If you using a longer reach plug than this one 2.5 to 3 threads is
optimum.
To increase the number of burnt threads increase the heat range of the plug, if you have 4-5-6 threads
burnt you need to get a colder plug."

 

The complete guide is posted elsewhere on the Faq.

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A radiator shop is a good place to take a leak.

 

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3 hours ago, jimk said:

They were also covered in carbon

If your engine has not been refreshed (valves, guides, rings) and is burning more oil than normal, you might try BP5 instead of BP6 plugs. These are a bit hotter and kept my plugs cleaner (in the self-cleaning temp range) longer.  

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